It is known from the prior art that anthraquinone may be obtained on heating with air or oxygen a solution or dispersion of octahydro-anthraquinone (French Pat. No. 673,825 of Apr. 22, 1929). Another process comprises heating 1, 4, 4a, 9a-tetrahydro-anthraquinone in the presence of nitrobenzene (British Pat. No. 895,620 of May 2, 1962). It is also possible to obtain anthraquinone by heating the 1, 4, 4a, 5, 8, 8a, 9a, 10a-octahydroanthraquinone in the presence of nitrobenzene, but the yield of the reaction is poor and the product obtained is not pure.
In addition, U.S. Pat. No. 4,152,340 discloses the production of anthraquinone by reacting 1, 4, 4a, 5, 8, 8a, 9a, 10a-octahydro-anthraquinone with nitrobenzene in the presence of a catalytic amount of a basic compound soluble in the reaction medium and an inhibitor of free radical reactions.
Those skilled in the art are constantly searching for new methods of preparation of anthraquinone due to this compound's versatility, however, none of the patents cited or other literature discloses or suggests a process in which a solvent is not employed nor is the use of a basic compound or inhibitor.
In addition, presently accepted methods of synthesis in the prior art discloses complex and costly separation and purification procedures which are not needed in the process of the present invention.
The inventors have made extensive investigations in an attempt to provide an improved process which can overcome the various disadvantages in the prior art suggestions and can separate and recover high purity anthraquinone in high yields by an industrially advantageous operation. These investigations finally led to the discovery that the various defects of the prior art can be overcome and high purity anthraquinone can be separated and recovered in high yields by an industrially advantageous operation.
The present invention encompasses an approach to anthraquinone by the catalytic dehydrogenation rather than the direct oxidation of octahydro-anthraquinone (OHAQ) intermediate or other intermediates. OHAQ is the reaction product of two molar equivalents of 1, 3-butadiene with one molar equivalent of benzoquinone and has the following structure: ##STR1##
To a chemist skilled in this particular art an acceptable approach would appear to be the dehydrogenation of OHAQ in the vapor phase by a continuous reaction over a fixed bed catalyst system. No solvent would be employed and the OHAQ would be metered into the system as a melt. This approach attempted by the applicants using several catalyst systems produced only traces of anthraquinone and the main reaction product as shown by mass spectroscopy, gas chromatography, NMR and IR analysis was anthracene with a melting point of 216.degree. C. The dehydrogenation is accompanied by dehydration over a palladium on an aluminum oxide carrier that gives an 83 percent yield of anthracene plus water according to the following reaction scheme: ##STR2##
In an attempt to overcome this undesirable reaction more inert catalysts were experimented with in the vapor phase. Low surface area alumina (Al.sub.2 O.sub.3) and Na.sub.2 CO.sub.3 /Al.sub.2 O.sub.3 gave anthracene as the major product, but also gave a complicated mixture of partially dehydrogenated and dehydrated products. Lower reaction temperatures were also attempted (less than 325.degree. C.) but would not properly vaporize products from the reactor system.
Attempts to use a solvent to moderate the reaction produced a variety of products that were dependent on the reaction conditions. The use of hydrogen acceptors in the solvent system did not appreciably improve the production of anthraquinone.
Liquid phase reactions of OHAQ without a solvent were carried out with OHAQ as a melt. The OHAQ and powdered catalysts were intimately mixed under nitrogen and heated above the melting point of OHAQ. When the catalyst was 5% palladium/carbon (Pd/C) a pronounced exotherm was observed near 230.degree. C. This exotherm is believed to be the heat of reaction of the isomerization of OHAQ to isomerized OHAQ (i-OHAQ) according to the following chemical structures: ##STR3##
Mass spectrum and NMR analysis of the product from this reaction showed that i-OHAQ to be an isomer of OHAQ and also indicated the absence of olefinic protons. The isolated i-OHAQ was then reacted at 273.degree. C. in the presence of a 5% palladium/carbon catalyst to affect the conversion to anthraquinone according to the following scheme: ##STR4##
The conversion of OHAQ to isomerized OHAQ is not without the formation of another intermediate product which is the isomer of tetrahydro-anthraquinone (THAQ): ##STR5##
The molecular weight of i-THAQ was verified by mass spec and the NMR analysis spectrum showed no olefin protons. A crude mixture of i-OHAQ and i-THAQ, approximately 1 to 1, gave a very similar yield of AQ which was obtained under the identical reaction conditions used to convert OHAQ to AQ but at a lower temperature, approximately 250.degree. C.
This fact suggests that the by-products, i-OHAQ and i-THAQ can be recycled.
This initial investigation also determined unexpectedly that the length of time that the melt reaction is held at or near 300.degree. C. has a pronounced effect on the yield of anthraquinone.
Other catalysts, such as rhodium, Rh/C and cobalt produce very little anthraquinone from OHAQ but gives high yields of the isomerized OHAQ. In addition, it was determined that used catalysts from a typical Rh/C run with OHAQ can be recycled to give almost identical results.
Tetrahydro-naphthoquinone (THNQ) is quite often an impurity in the preparation of OHAQ and it was determined that a 1 to 1 mixture of OHAQ and THNQ gave acceptable yields of anthraquinone and naphthoquinone when short reaction times were employed after the initial exotherm of the melt.